Passive lowpass, highpass, bandpass, and bandreject filters, including radio frequency (RF) filters, are commonly used in electronic equipment. Communications equipment in particular relies on the extensive use of passive filtering to aid in the extraction of a desired signal from noise and interference, to ensure spectral purity of transmitted signals, and other uses.
Multiband designs may use large numbers of switchable passive filters to make recovery of the desired signal feasible, economical, or to provide enhanced performance. Some switchable passive filters use varactors as the main tuning component, and several types of active filters have been suggested (i.e., gmC and logarithmic) but they all suffer from dynamic range and current drain limitations when compared to passive filter counterparts.
Filter hardware suitable for a Software Defined Radio (SDR) in general needs to be frequency agile. In order to be most useful, the hardware filters typically must be able to cover a wide bandwidth and be capable of providing various bandwidths at a particular operating frequency within a given frequency range of interest. Common radio applications require both wideband and narrowband filters, and the filter frequency of operation which is required depends on the radio design and the point of use of the filter within the radio.
SDR applications also require that properties of hardware bandpass and bandstop filters, such as center frequency and bandwidth, be controllable by software/digital means. Similarly, where highpass filtering is employed it is desirable that the highpass filtering have a selectable corner frequency under software control. Prior art flexible lowpass RF filters are incapable of meeting this flexible highpass RF filtering requirement.
No truly satisfactory solution to this requirement exists in the prior art. What is needed is a method and apparatus for creating a filter that has flexibility in corner frequency selection, and maintains the low current drain and high dynamic range performance of passive filters.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.